[RFC PATCH v5 5/6] rust: DmaFence: Replace call_rcu() with synchronize_rcu()

[Philipp Stanner]

The C dma_fence contract demands that a fence's data disappears no
earlier than 1 RCU grace period after the DriverFence was signaled.

So far, we achieved this with dropping the DriverFence's data in a
deferred manner with a manual, raw binding call to call_rcu().

However, this call_rcu() does not solve one problem: It is conceivable
that some driver data must not drop in atomic context; but call_rcu()'s
payload *can* be executed in atomic context.

Moreover, the current developments in drm::JobQueue, where a DriverFence
is always coupled 1:1 with a Job, allows for dropping job and fence
through a work item.

The undesirable blocking behavior of synchronize_rcu() would, thus, not
be annoying any thread anymore. The direct users of DmaFence could
achieve the same by dropping their DriverFences through work items.

The only conceivable other solution, queue_rcu_work(), would, for many
users, schedule a work_item from another work_item, which seems
undesirable.

Additionally, using the existing synchronize_rcu() abstraction has the
advantage of us getting rid of a raw bindings call.

Replace DriverFence::drop()'s call_rcu() with synchronize_rcu().

Signed-off-by: Philipp Stanner <phasta@xxxxxxxxxx>
---
 rust/kernel/dma_buf/dma_fence.rs | 71 +++++++-------------------------
 1 file changed, 14 insertions(+), 57 deletions(-)

diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs
index 4e9b4a6a9471..cefcde80bf09 100644
--- a/rust/kernel/dma_buf/dma_fence.rs
+++ b/rust/kernel/dma_buf/dma_fence.rs
@@ -42,7 +42,8 @@
             Atomic,
             Relaxed, //
         },
-        rcu::rcu_barrier, //
+        rcu::rcu_barrier,
+        rcu::synchronize_rcu, //
     }, //
 };
 
@@ -150,7 +151,6 @@ pub fn new_fence_allocation(
         data: T::FenceDataType,
     ) -> Result<DriverFenceAllocation<'_, T>> {
         let fence_data = DriverFenceData {
-            rcu_head: Default::default(),
             // `inner` remains uninitialized until a `DriverFence` takes over.
             inner: Fence {
                 inner: Opaque::uninit(),
@@ -640,8 +640,6 @@ struct DriverFenceData<'a, T: Send + Sync + FenceContextOps> {
     // necessary so that the C backend can free the allocation (coming from our
     // Rust code) with kfree_rcu().
     inner: Fence,
-    /// Callback head for dropping this in a deferred manner through RCU.
-    rcu_head: bindings::callback_head,
     /// Reference to access the FenceContext.
     fctx: &'a FenceContext<T>,
     /// The API user's data. It is essential that the data only performs
@@ -1022,59 +1020,18 @@ fn drop(&mut self) {
             return;
         }
 
-        // SAFETY: Valid because `self` is valid.
-        let rcu_head_ptr = unsafe { &raw mut (*self.data.as_ptr()).rcu_head };
+        // Make sure none of the fence backend_ops can access data anymore.
+        //
+        // TODO:
+        // This would not be necessary if the C dma_fence backend were using a
+        // spinlock to properly synchronize its signaled state. Fix it in C and
+        // then remove synchronize_rcu().
+        synchronize_rcu();
 
-        // SAFETY: `call_rcu()` is always safe to be called. `rcu_head_ptr` was
-        // created validly above. The module must perform a `synchronize_rcu()`
-        // or `rcu_barrier()` call to guard against module unload.
-        unsafe { bindings::call_rcu(rcu_head_ptr, Some(drop_driver_fence_data::<T>)) };
+        // SAFETY: Valid because `self` is valid.
+        unsafe { drop_in_place(&raw mut self.data) };
+
+        // SAFETY: The `synchronize_rcu()` above ensures all accessors are gone.
+        unsafe { bindings::dma_fence_put(self.as_raw()) };
     }
 }
-
-// TODO:
-// The entire call_rcu() mechanism in the drop above and the code below would be
-// unnecessary if C's dma_fence_signal() could be reworked in a way that after it
-// ran, the caller knows that no fence_ops callbacks can be running anymore.
-// In other words, if the dma_fence backend would use its spinlock for full
-// synchronization.
-//
-// Then we could move the drop_in_place() and dma_fence_put() upwards into the
-// drop() implementation and call it a day.
-
-/// Finally really drop this `DriverFence<T>`
-///
-/// # Safety
-///
-/// `head` references the `rcu_head` field of an `DriverFenceData<T>`. All
-/// accessors to that `DriverFenceData<T>` must be gone by now. This must be
-/// ensured by signalling the associated `DriverFence<T>` and then waiting
-/// for a grace period until calling this function here.
-unsafe extern "C" fn drop_driver_fence_data<T: Send + Sync + FenceContextOps>(
-    head: *mut bindings::callback_head,
-) {
-    // SAFETY: Caller provides a pointer to the `rcu_head` field of a `DriverFenceData<C>`.
-    let fence_data = unsafe { container_of!(head, DriverFenceData<'_, T>, rcu_head) };
-
-    // SAFETY: `fence_data` was created validly above. All the fence's data will
-    // only drop below, but the raw pointer to the raw C `dma_fence` remains
-    // valid because the reference count is only decremented at the end of the
-    // function.
-    let fence = unsafe { (*fence_data).inner.inner.get() };
-
-    // SAFETY: `fence_data` was created validly above. The user has already
-    // dropped the only conventional accessor to the user data, the `DriverFence`,
-    // one grace period ago. All accessors are gone now.
-    unsafe { drop_in_place(&raw mut (*fence_data).data) };
-
-    // The inner `Fence` explicitly does not get dropped because there may be
-    // many more users / consumers, each holding their own reference.
-
-    // SAFETY: Once a `DriverFence` is initialized, the inner `fence` is valid
-    // and initialized. It is valid until the refcount drops to 0, which can
-    // earliest happen once we drop the `DriverFence`'s reference here.
-    unsafe { bindings::dma_fence_put(fence) };
-
-    // The actual memory the data associated with a `DriverFence` lives in
-    // gets freed by the C dma_fence backend once the fence's refcount reaches 0.
-}
-- 
2.55.0